1. Field of the Invention
The present invention relates to image enhancement, and more specifically, to a method and apparatus for enhancing images using variable plateau histogram equalization.
2. Related Art
In various imaging applications, such as military imaging applications, it is often desirable to enhance an image acquired by a camera or sensor to reveal additional detail in the image. For example, where images are taken in the presence of a bright light, details in shadows and in bright areas are often lost. Likewise, details are often lost in dark areas of dark images. As such, it is desirable to reveal such details without compromising overall image quality.
A number of image enhancement methods have, in the past, been developed. Examples of such methods include contrast stretching, histogram equalization, histogram projection, and plateau equalization. In contrast stretching, pixel intensity distributions are expanded (“stretched”) to occupy a full range of possible values. According to this method, minimum and maximum pixel intensity values are measured (or estimated), and offset and gain correction values are applied to the pixel intensity values corresponding to each pixel in the image, so that the full range of intensity values is occupied. In histogram equalization, the pixel intensity distribution is attenuated or flattened. According to this method, an input histogram is constructed, which includes the total number of pixels corresponding to each intensity level within the image. Then, a cumulative histogram is constructed and normalized, and each pixel in the image is transformed using the normalized cumulative histogram as a look-up table. In histogram projection, a histogram is constructed, consisting of occupied and unoccupied levels. A cumulative histogram is then constructed and normalized, and each pixel in the image is transformed using the normalized cumulative histogram as a look-up table. In plateau equalization, an input histogram is constructed and clipped at a specified level or “plateau.” Then, a cumulative histogram is constructed and normalized, and each pixel in the image is transformed using the normalized cumulative histogram as a look-up table.
The foregoing image enhancement methods suffer from a number of drawbacks. For example, existing image enhancement methods do not adequately reveal detail in specific regions of an image, such as shadows or bright areas. Rather, these methods uniformly enhance an entire image, which may result in distortion of other regions of the image for the sake of enhancing a desired region of the image. Further, existing image enhancement methods cannot be easily adapted, without manual adjustment, to enhance images in different environments (e.g., by dynamically altering enhancement characteristics in response to changes in camera sensitivity).
Accordingly, what would be desirable, but has not yet been provided, is a method and apparatus for enhancing images which addresses the foregoing limitations of existing image enhancement methods.